OTOH, my point, Apollo, was that you should finally understand that peoples like you, who trying to make all theirselves, as cheaper as possible, are quite quite rare. I know only one such person - you. I believe we should give small manufacturers a freedom to make the things the way they see. They always compete each other. Their overall view on a subject/problem usually wider and smarter than one single person's point of view.

And Oliver's solution is best on the market. The solution you offer is exactly the same as Coronado does/doing and now these filters should be replaced. Oliver's filters will work decades. Good enough, I believe.

I will buy Oliver's ITF filter despite the ability to make such a filter myself. The best way is: we need to earn money doing what we can do best - at our job or extra job, but still where we are good. Then buy the things we need and which we can't do right. Unless we are peoples like you are.

"Solar H alpha activity is the most dynamic and compelling thing you can see in a telescope, so spend accordingly." (c) Bob Yoesle.

The ITF of the solar blocking filters do not fail from lamination separation or de-contacting between the filter components, they fail from thermal cycling and loading. This compromises the edge seal, which results in atmospheric moisture being able to penetrate the filter and degrade the silver layers of the ITF coating. Oliver’s solution therefore completely removes this silver layer deterioration mechanism.

Secondly, the purpose of the ITF is mostly long IR blocking out to about 4000-5000 nm, and why Oliver’s solution is to incorporate the KG3 element instead of the silver layer ITF. Although the ITF also incorporates a RG630 substrate for UV blocking, this long IR blocking is something the standard nighttime H alpha RG filter with standard UV/IR blocking to 1200-1400 nm cannot do.

However, if one already has an RG630 element elsewhere in the system, preferably also with standard IR blocking to 1200-1400 nm such as the ERF’s used by Lunt and Solar Scope, one then does not need the RG630 element in the ITF portion of the blocking filter. Solar Scope therefore has eliminated the ITF from their blocking filter(s), and so has Lunt. Solar Scope uses a KG filter instead, and Lunt uses a BG element.

Since my systems incorporate RG630 ERF elements, I have used Oliver’s other ITF replacement filter - a KG3 UV/IR filter - to replace a rusted BF30 ITF with great results and improved image brightness.

Diagonally parked in a parallel universe.

Curiosity is the father of knowledge; uncertainty is the mother of wisdom.

I’ve mentioned previously, but repeat my myself again.
Why do we think blocking IR beyond 1400nm is an important issue in solar filter design?
This region is heat not light and well beyond the sensitivity of the eye or current imaging sensors.
I accept that extended and prolonged exposure to IR can be harmful, but this is not the case with the average solar observer.

"This region is heat not light and well beyond the sensitivity of the eye or current imaging sensors."

There appear to be other structures of the eye besides the retina that are susceptible to mid and long IR:

Occular absorbtion.jpg (162.2 KiB) Viewed 912 times

Here's what our friend Mark Wagner of Solar Spectrum filters had to say in this regard:

The ITF is used to block what the main bandpass doesn't, the one that only lets one peak from the etalon through. The bandpass will normally turn back on at around 800 nm. So the ITF is designed to block any light from 690 nm to the FIR (past 2500 nm). It is not uncommon to be still blocking past 3000 nm. They will use a red glass (RG630) to help block the short side (lower wavelengths). These filters will be spec’d as blocked from X-ray to FIR with a of OD4. This is how a standard bandpass keeps the level of IR low for safe viewing.

Filters that are blocked to only 1000 nm (like most stock filters) are not safe for viewing the sun, but maybe fine for imaging depending on the camera. To decide how far in the IR is safe for viewing I would say nothing shorter then 2000 nm. I wanted to see what a filter looked like with a dim etalon that I only blocked to about 900 nm. I did this test because the hard coated filters I use have more transmission than a standard one. And it would be closer in brightness. I only looked for about 5 minutes. When I was done I noticed that I couldn't focus the eye I had been using. It took about 45 minutes before I could focus with that eye. Didn't get hurt but got scared that I had damaged my eye. So don't try viewing with filters that are blocked anything shorter then 2000 nm on the sun.

KG glass is also used in blocking the IR.

Mark's description sounds very similar to what one expects with corneal edema (swelling) and irritation likely from IR exposure.

George9 followed up with a well-regarded expert in eclipse filter safety:

Thanks, Mark. Dr. Ralph Chou was nice enough to answer a couple of email questions in this regard, too. He let me quote him here.

First, with regard to solar eclipse eyeglasses: "You are correct that the specification for solar filters does not need to go beyond 1500 nm since water within the ocular media prevent longer wavelengths from reaching the retina. As for the cornea, the thresholds for damage by infrared radiation are extremely high, and even with a telescope, the danger to the cornea from IR between 1500 and 2500 nm while observing the Sun is not significant. The solar filter materials do block IR quite well out to about 3000 nm, so the eye is well protected across the entire UV-visible- NIR waveband." There is an existing eclipse eyeglass standard (EN1836) and a new draft ISO standard (ISO 12312-2), for which Dr. Chou is lead writer.

But with regard to telescope filters: "I expect that the manufacturers are well aware that they need to ensure that all of the optical waveband in sunlight (280 to 3000+ nm) has to be attenuated for eye safety, no matter what their technology is."

In other words, while the cornea is relatively resistant to damage in the 1500-3000nm range and should be fine without magnification, you still really need to make sure it is blocked in a telescope. And it would be possible to create a filter that is perfectly safe for solar eyeglasses but would not be a great idea to use by itself in front of a large telescope.

If we do a little math, we can see that my 4 inch (102 mm) telescope will present 650 times the amount of light (hence IR) than the unaided (naked eye) alone:

Applying this methodology to H alpha filters, we can see there is also potential here as well. For example, if the etalon has a free spectral range of 10 Å, and a FWHM (50%) transmission of 0.7 Å, the native 650 times the pupil transmission would be:

0.7/10 = 0.07
0.07 x 650 = 45.5 x 0.50 = 23

So if a solar H alpha blocking filter is not blocked at all beyond 1400 nm, in the case of a 102 mm refractor it could present the eye with about 23 times as much mid and far IR as the pupil would receive alone, and additionally is focused at the eye itself. And who is to say what an individuals specific susceptibility to any form of radiation could be based on their genetics and previous exposures, which generally are cumulative?

"I accept that extended and prolonged exposure to IR can be harmful, but this is not the case with the average solar observer."

Why take the chance, especially when a KG3 filter (or ITF, BG38, etc.) is relatively inexpensive insurance? And if for no other reason, in our litigious society, it wouldn't take much for a casual observer at one of my solar outreach events to show up and claim my solar filters were not safe and caused their eye issues. We go to court, and I state because Ken Harrison said it was safe I didn't use any IR blocking beyond 1400 nm. The plaintiff's attorney points out with documentation that every solar filter and H alpha telescope manufacturer incorporates IR blocking to 3000 nm or more. He or she also quotes the above information from Wagner and Chou, readily discoverable on the internet. The plaintiffs then ad you to their lawsuit for contributory liability. Who do you think the jury or judge is more likely to side with?

If you truly believe there is no harm from 1400 nm and beyond, this should be the acid test:

This filter is blocked OD 4 from 200 nm to the cut-on of 1400 nm. Focus your equatorial mounted 6 inch f 8 refractor on the sun using a standard piece of Baader astrosolar filter material. Add the FEL 1400 to your eyepiece, remove the Baader astrosolar material, and "observe" only with your non-dominate eye for couple of hours every week for a year or two, and let us know how it goes. I hope you are right.

Diagonally parked in a parallel universe.

Curiosity is the father of knowledge; uncertainty is the mother of wisdom.

Individual biological tissue susceptibility, dose, exposure time, and recovery time, are all variables which need to be accounted for with any kind of energy exposure, whether it is electromagnetic, acoustic, or kinetic. With IR there seems to be a paucity of data related to optical instruments in general, and even more so related to telescope observation and specific solar telescope and filter systems:

While I consider myself to be a solar enthusiast, I am by no means an expert when it comes to solar equipment and safety. My perspective is that I generally don’t have enough knowledge or information about what we are dealing with to make perfectly informed decisions. Filter components and coatings change between companies, over time, and within product lines. Hopefully the amount of IR B for most solar telescopes and filter systems falls well within acceptable limits. However, stating there is no concern at all for IR B did not seem “reasonable,” especially when others who have some expertise with solar radiation apparently believe IR blocking beyond 1400 nm is important at least to some degree for safety.

Moreover, you can not use qualitative marketing verbiage to make definitive or quantitative conclusions about specific filter components. While we do have some dated independent data about the Coronado ERF (no IR blocking), Baader ERF (IR blocking to ~ 1400 nm), and Solarscope ERF (IR blocking to ~ 1000 nm), and the Coronado ITF (blocks to 2500 + nm) http://www.sonnen-filter.de/index-x.html , we don’t have specifics on what the current Lunt ERF or other components transmit or block to, other than the Schott catalog for the BG and KG filters.

So the fact that Lunt now uses a BG element instead of an ITF, and states that the BG element takes care of “residual” unspecified IR and may be redundant actually may mean little. Maybe Lunt has placed all needed UV and IR blocking ahead of the blocking filter diagonal. But we don’t know for sure because we lack sufficient information about the filter system components and their transmission characteristics, and have only rather vague descriptions that an unspecified level of blocking to 3000 nm is needed for telescopic (not naked eye/eclipse filters) observation.

I do know for a fact that etalons can allow IR harmonic peaks to pass. I used an ERF/etalon and blocking filter system from a very highly regarded solar filter manufacturer, which produced noticeably defective images. It turned out that the blocking filter was letting IR A from the ERF/etalon through, which was invisible to the eye but quite noticeable from imaging:

IR leak.jpg (218.44 KiB) Viewed 814 times

Top image H alpha filtergram with out-of-focus IR A leakage, bottom same system with added IR blocking filter.

Whether or not this IR was significant or posed a safety issue I could not tell, although I did not observe for more than a minute or two at a time. Luckily, my 100ED semi-APO telescope couldn’t focus the IR A with the H alpha. Had I used a “better” APO that could, the situation might have been more serious. A simple IR blocking filter cured the problem, and after I supplied photographic documentation, the manufacturer quickly remedied the situation for me and future customers.

This experience, and my background in health care ("if something bad can happen it will happen - to somebody"), make the bottom line easy for me. When it comes to solar radiation and unknown amounts hitting and entering my eye, I believe prudence is warranted - and to paraphrase - a gram of prevention is worth a kilogram of cure.